1. Field of the Invention
The present invention relates to a wavelength converter which generates from an incident laser light beam a second harmonic of the laser light beam, and is easily positioned within a light source device and/or outputs the second harmonic laser light as parallel rays.
2. Description of the Related Art
Second harmonics of laser light occur in a medium due to the nonlinear optical effect. This phenomenon causes the generation of second harmonics when the polarization of laser light incident on a medium is proportional to the second and higher degrees of the electric field of the incident laser light.
Materials in which the nonlinear optical effect occurs are called nonlinear optical materials. The inorganic materials KH.sub.2 PO.sub.4 and LiNbO.sub.3 are currently used nonlinear optical materials. Organic material 2-methyl-4-nitrileanilin (MNA) has attracted some attention as a nonlinear optical material because it has a superb nonlinear optical constant.
Typical wavelength converter confine the fundamental component (i.e. incident laser light) using high energy density nonlinear optical materials, wherein the interaction of the harmonic components and the fundamental components is elongated. For this reason, an optical wave guide type of wavelength converter is used. This type of wavelength converter has a narrow wave guide, within which light propagates, formed on a substrate; and the wave guide and substrate are covered with an overlayer. In order to produce copious amounts of second harmonic light, the optical wave guide must accept a phase propagating velocity of the second harmonic. In other words, the fundamental component must phase match the second harmonic component. The simplest method known for obtaining this phase match is based on Cerenkov radiation.
The Cerenkov method of phase matching follows. Shown in FIG. 1 (PRIOR ART), a second harmonic light is generated from light propagating through an optical guide portion 121 at point A. The second harmonic light propagates at and angle .alpha., with respect to the optical axis of the optical guide portion 121, towards a substrate 122 and an overlayer 123. After a predetermined time period, the second harmonic light is generated at point B as generated at point A. If the equiphase plane of the second harmonic light generated at point A is coincident with that of the second harmonic light generated at point B, then the second harmonic light is emitted. The emission of the second harmonic light is in the direction of angle .alpha.. If the following relation holds: EQU n.sub.S (2w)&gt;n.sub.G (w)&gt;n.sub.S (w), (1)
where,
n.sub.S (w) is the refractive index of the substrate 122 for the fundamental component, PA1 n.sub.G (w) is the refractive index of the wave guide portion 121, and PA1 n.sub.S (2w) is the refractive index of the substrate 122 for the second harmonic component;
phase matching occurs.
In an optical wave guide type of wavelength converter as described above, the flux of light rays propagating through the wave guide is accurately shaped in cross section; however, condensation of the emitted light rays is poor. In other words, the emitted light rays cannot be condensed into a small spot. Therefore, it is difficult to utilize the second harmonic light for writing data into and reading data out of an optical recording medium, such as an optical disk.
It has been demonstrated that an optical fiber type of wavelength converter as shown in FIG. 2 can realize a high density recording in an optical disk. Wavelength converter 130 of the optical fiber type includes a core 131 and a clad 132 having refractive indices satisfying equation (1). The second harmonic light 133 emitted from the end face expands in the form of a rotational symmetric ring. Accordingly, this type of wavelength converter has an excellent condensing characteristic.
To condense the emitted light rays of a wavelength converter of the optical fiber type, it is necessary to collimate (make parallel) the emitted light rays. Japanese Patent Unexamined Publication Nos. Hei.1-287531, 1-293325, 2-35423, 2-153328, and 2-186327 disclose techniques to collimate emitted light rays.
In Japanese Patent Unexamined Publication No. Hei. 1-287531, there is disclosed a light source device in which a circular cone prism, used as a collimating lens, collimates second harmonic light emitted from a wavelength converter of the optical fiber type.
Shown in FIG. 3, is the technique disclosed in Japanese Patent Unexamined Publication No. Hei.2-153328 for collimating emitted light rays using a Fresnel lens 151. The Fresnel lens 151 incorporates a concentric circular diffraction grating directed towards the light emitting end face 150a of wavelength converter 150. Once the emitted light rays have been collimated the light rays can be easily condensed.
In both devices described above, it is difficult to position the wavelength converter within a light source device due to the small size (1 to 2 .mu.m) of the wavelength converter. The devices also have the disadvantage that it is difficult to properly align the optical axis of the wavelength converter with the axis of rotational symmetry of the collimating lens. Furthermore, difficulty arises in the correct adjustment of the distance between the light emitting end face of the wavelength converter and the collimating lens; exact positioning is quite complicated.
The device disclosed in Japanese Patent Unexamined Publication No. Hei. 1-293325 is shown in FIG. 4. As shown, the light emitting end face of the optical fiber 141 is machined to a circular slanting surface 141a for collimating emitted second harmonic light.
This device has the drawback that the light emitting end face must be accurately machined. Often in machining, the light emitting end face is broken or scarred; thus reducing the amount of second harmonic light emitted.